1. Field of the Invention
The present invention relates generally to a method for recovery water which is safe for drinking from seawater. More particularly, the present invention relates to a method which uses an ammonia bicarbonate draw solution in a forward osmosis process to desalinate seawater to provide safe drinking water.
2. Description of the Prior Art
Safe drinking water is a vital commodity in military operations that must be supplied to troops on a continuing basis. Potable water carried by a typical soldier or Marine in a combat environment accounts for about forty percent by weight of the consumable material which the soldier or Marine must carry.
Potable water for military operations is often made in large quantity from local supplies of fresh, brackish or salt water using the reverse osmosis (RO) process.
The military has used RO technology for years for water purification with great success. However, reverse osmosis technology is significantly more complex than conventional municipal processes for producing potable water. In addition, RO is often more energy intensive than conventional municipal processes. Reverse osmosis requires pressurizing seawater to a very high pressure, (typically 900 to 1000 pounds per square inch) for operation. For large commercial reverse osmosis installations, the energy consumption of the process can be reduced by installing devices that recover a significant of the energy expended to pressurize the seawater. For military operations, however, the complexity, weight, and maintenance requirement of these energy recovery devices usually precludes their use.
Forward osmosis is a process used to desalinate seawater and produce potable water from non-potable sources including saltwater. This process produces potable water through a semi-permeable membrane, such as a membrane assembly, that allows the passage of water but blocks the passage of salts. In forward osmosis, the driving force that drives water across the membrane is the difference in osmotic pressure across the membrane. Forward osmosis draws the water molecules through the membrane using a difference in osmotic pressure.
For forward osmosis, seawater is on one side of the membrane and a draw solution is on the other side of the membrane. The draw solution has an osmotic pressure that is significantly higher than that of seawater. This allows the draw solution to draw water molecules from the seawater through the membrane in an effort to equalize the osmotic pressure on both sides of the membrane.
For forward osmosis to be effective in drawing water molecules from seawater, the draw solution must have an osmotic pressure greater than that of seawater. When seawater is approximately 3.5% NaCl, the molarity of sodium chloride is equal to 0.60 moles/liter. In accordance with Van't Hoff's equation, the osmotic pressure is:Π=I*C*R*Twhere    Π=osmotic pressure    I=Van't Hoff's factor    C=molar concentration, moles/liter    R=universal gas constant=0.08206 Liter*atm/mole/° K    T=° KThen the osmotic pressure for seawater is:Πseawater=1.8*0.60*0.08206*26.4=26.4 atm
Many draw solutions can be identified which would function in the present invention. However, there is a need to identify a non-toxic compound that is easily separated from the draw solution so that only potable water remains. The ideal draw solution consist of a chemical compound or a nanometer size particle that is easily removed from a diluted draw solution and reused. Draw solutions may include magnetic proteins (magnetoferritin) particles, liquids that can easily be separated from water by low temperature distillation, and compounds that sublime to gases.
A chemical compound that sublimes to gases is ammonium bicarbonate (NH4HCO3). Ammonium bicarbonate is a non-toxic, low cost, industrial chemical used as a rising agent in commercial bakeries. It is also used as a fertilizer. The most useful characteristic of ammonium bicarbonate for the present invention is that it decomposes into ammonia gas, carbon dioxide, and water vapor at temperatures above 150° F. This property of ammonium bicarbonate makes it a very desirable candidate for use as a draw solution in the present invention.